Multi-path compensation using multiple modulation frequencies in time of flight sensor

Abstract

A method to compensate for multi-path in time-of-flight (TOF) three dimensional (3D) cameras applies different modulation frequencies in order to calculate/estimate the error vector. Multi-path in 3D TOF cameras might be caused by one of the two following sources: stray light artifacts in the TOF camera systems and multiple reflections in the scene. The proposed method compensates for the errors caused by both sources by implementing multiple modulation frequencies.

What is claimed is: 
     
       1. A time of flight three dimensional imaging system, comprising:
 an illumination module that generates modulated light that is intensity modulated at a first frequency and at a second frequency; 
 a sensor that detects the modulated light at the first frequency and at the second frequency; and 
 a controller that generates a three dimensional image from the detected modulated light at the first frequency and at the second frequency and compensates for multipath error in the three dimensional image arising from the modulated light traveling an indirect path before detection based, at least in part, on a relationship between (i) a ratio of the generated modulated light at the first frequency and the generated modulated light at the second frequency, and (ii) a ratio of the detected modulated light at the first frequency and the detected modulated light at the second frequency. 
 
     
     
       2. A system as claimed in  claim 1 , wherein the controller determines a vector associated with the multipath error by estimating a phase and an amplitude of the modulated light that traveled the indirect path and corrects a depth measured by the controller based on the vector. 
     
     
       3. A system as claimed in  claim 2 , wherein the controller uses iterative approximations to determine the vector. 
     
     
       4. A time of flight three dimensional imaging method, comprising:
 generating modulated light that is intensity modulated at a first frequency and at a second frequency; 
 detecting the modulated light at the first frequency and at the second frequency; 
 generating a three dimensional image from the detected modulated light; and 
 compensating the three dimensional image for multipath error arising from the modulated light traveling an indirect path before detection based, at least in part, on a relationship between (i) a ratio of the generated modulated light at the first frequency and the generated modulated light at the second frequency, and (ii) a ratio of the detected modulated light at the first frequency and the detected modulated light at the second frequency. 
 
     
     
       5. A method as claimed in  claim 4 , further comprising determining a vector associated with the multipath error by estimating a phase and an amplitude of the modulated light that traveled the indirect path and correcting a depth based on the vector. 
     
     
       6. A method as claimed in  claim 5 , further comprising using iterative approximations to determine the vector. 
     
     
       7. A system as claimed in  claim 1 , wherein the illumination module generates the modulated light at the first frequency and the second frequency serially in time. 
     
     
       8. A system as claimed in  claim 1 , wherein the illumination module generates the modulated light at the first frequency and the second frequency simultaneously but at different optical wavelengths. 
     
     
       9. A method as claimed in  claim 4 , further comprising generating modulated light at the first frequency and the second frequency serially in time. 
     
     
       10. A method as claimed in  claim 4 , further comprising generating the modulated light at the first frequency and the second frequency simultaneously but at different optical wavelengths.